1. Field of the Invention
The present application relates to the field of oversampled, noise-shaping signal processing which includes, for example, sigma-delta modulation technology. More specifically, the present invention provides methods and apparatus for maintaining a balance between the amount of delay compensation in a modulator loop and the average switching frequency of the loop.
2. Description of the Prior Art
The THD+noise vs. Vout curve shown in FIG. 1 characterizes many modulator loops such as for example, sigma-delta modulator loops and the modulator loops described in U.S. Pat. No. 5,777,512 for METHOD AND APPARATUS FOR OVERSAMPLED, NOISE-SHAPING, MIXED-SIGNAL PROCESSING issued on Jul. 7, 1998, the entire disclosure of which is incorporated herein by reference for all purposes. The position of the xe2x80x9celbowxe2x80x9d or xe2x80x9ckneexe2x80x9d 100 of the curve, i.e., the point at which the slope of the curve increases dramatically, is, in part, a function of the amount of delay in the feedback loop of the modulator. That is, the greater the amount of delay in the loop, the further to the right the elbow moves.
U.S. Pat. No. 5,909,153 for METHOD AND APPARATUS FOR COMPENSATING FOR DELAYS IN MODULATOR LOOPS issued on Jun. 1, 1999, the entire disclosure of which is incorporated herein by reference for all purposes, describes a technique by which feedback from the output of the comparator is introduced at various points in the loops integrator stages. These delay compensation technique also have the effect of moving elbow 100 to the right. In that application, an estimate of the loop delay is made and a fixed amount of compensation feedback which corresponds to the estimate is provided.
However, if the elbow if pushed too far out, distortion is introduced in the band of interest and the THD+noise floor goes up. This may be caused, for example, by the fact that some of the feedback information is getting into the band of interest or that the feedback causes the loop to behave in a way which doesn""t model the original transfer function ideally. That is, because there are a number of nonlinearities in the loop, if the loop behavior and/or the average switching frequency is modified, the distortion characteristics of the various loop elements are commensurably modified which ultimately introduces more distortion.
Thus, what is desired is an optimal delay compensation since as the amount of delay in the loop is increased, more compensation is needed which, in turn, adversely affects the linearity of the circuit.
The present invention monitors the average switching frequency at the output of the comparator and adjusts the delay compensation accordingly. That is, the switching frequency monitoring circuit looks at the average switching frequency and maintains the average switching frequency in a xe2x80x9cfrequency zonexe2x80x9d which corresponds to a high elbow and a low noise floor.
In one embodiment, an apparatus for providing a loop delay to a modulator loop circuit is disclosed. The apparatus includes a switching frequency monitoring circuit coupled to the loop circuit arranged to determine a current switching frequency of the loop circuit, and to generate control signals based upon the current switching frequency. A delay compensation circuit is coupled to the loop circuit and the switching frequency monitoring circuit arranged to provide the loop delay to the loop circuit based upon the control signals so as to maintain a balance between an amount of delay compensation in the loop circuit and an average switching frequency of the loop circuit.
In another embodiment, a method of providing a loop delay to a modulator loop circuit. The method is carried out by determining a current switching frequency of the loop circuit, generating control signals based upon the current switching frequency, and providing the loop delay to the loop circuit based upon the control signals so as to maintain a balance between an amount of delay compensation in the loop circuit and an average switching frequency of the loop circuit.